Patch "block: fix module reference leak on put_disk() call for cgroups throttle" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

block: fix module reference leak on put_disk() call for cgroups throttle

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
block-fix-module-reference-leak-on-put_disk-call-for-cgroups-throttle.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Roman Pen <[email protected]>
Date: Tue, 9 Feb 2016 12:33:35 -0700
Subject: block: fix module reference leak on put_disk() call for cgroups throttle

From: Roman Pen <[email protected]>

commit 39a169b62b415390398291080dafe63aec751e0a upstream.

get_disk(),get_gendisk() calls have non explicit side effect: they increase the reference on the disk owner module.

The following is the correct sequence how to get a disk reference and to put it:

    disk = get_gendisk(...);

    /* use disk */

    owner = disk->fops->owner;
    put_disk(disk);
    module_put(owner);

fs/block_dev.c is aware of this required module_put() call, but f.e. blkg_conf_finish(), which is located in block/blk-cgroup.c, does not put a module reference. To see a leakage in action cgroups throttle config can be used. In the following script I'm removing throttle for /dev/ram0 (actually this is NOP, because throttle was never set for this device):

    # lsmod | grep brd
    brd                     5175  0
    # i=100; while [ $i -gt 0 ]; do echo "1:0 0" > \
        /sys/fs/cgroup/blkio/blkio.throttle.read_bps_device; i=$(($i - 1)); \
    done
    # lsmod | grep brd
    brd                     5175  100

Now brd module has 100 references.

The issue is fixed by calling module_put() just right away put_disk().

---
 block/blk-cgroup.c |    9 +++++++++
 1 file changed, 9 insertions(+)

@@ -788,6 +788,7 @@ int blkg_conf_prep(struct blkcg *blkcg,
 {
 	struct gendisk *disk;
 	struct blkcg_gq *blkg;
+	struct module *owner;
 	unsigned int major, minor;
 	int key_len, part, ret;
 	char *body;

@@ -804,7 +805,9 @@ int blkg_conf_prep(struct blkcg *blkcg,
 	if (!disk)
 		return -ENODEV;
 	if (part) {
+		owner = disk->fops->owner;
 		put_disk(disk);
+		module_put(owner);
 		return -ENODEV;
 	}
 

@@ -820,7 +823,9 @@ int blkg_conf_prep(struct blkcg *blkcg,
 		ret = PTR_ERR(blkg);
 		rcu_read_unlock();
 		spin_unlock_irq(disk->queue->queue_lock);
+		owner = disk->fops->owner;
 		put_disk(disk);
+		module_put(owner);
 		/*
 		 * If queue was bypassing, we should retry.  Do so after a
 		 * short msleep().  It isn't strictly necessary but queue

@@ -851,9 +856,13 @@ EXPORT_SYMBOL_GPL(blkg_conf_prep);
 void blkg_conf_finish(struct blkg_conf_ctx *ctx)
 	__releases(ctx->disk->queue->queue_lock) __releases(rcu)
 {
+	struct module *owner;
+
 	spin_unlock_irq(ctx->disk->queue->queue_lock);
 	rcu_read_unlock();
+	owner = ctx->disk->fops->owner;
 	put_disk(ctx->disk);
+	module_put(owner);
 }
 EXPORT_SYMBOL_GPL(blkg_conf_finish);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/block-fix-module-reference-leak-on-put_disk-call-for-cgroups-throttle.patch